The present invention relates to inserts of cemented carbide bodies and rock drill bits preferably for percussive rock drilling.
In U.S. Pat. No. 4,598,779 is shown a rock drill bit that is provided with a plurality of chisel-shaped cutting inserts. Each insert discloses a guiding surface that is relatively sharply connected to cutting edges. A relatively sharp connection is disadvantageous when using cemented carbide that is extra hard. That is, flaking will occur during severe rock drilling due to tension in the connections, such that straight holes may not be achieved in the long run. Also the shape of the known insert is not optimized for maximum wear volume. U.S. Pat. No. 4,607,712 discloses a rock drill bit which has a plurality of cutting inserts. The working part of each insert has a semispherical basic shape, to which has been added extra volume of cemented carbide. However, the prior art insert does not sufficiently support against the wall of the bore such that straight holes may not be achieved. Furthermore, connections between the components of the working part are relatively sharp thereby producing the above-mentioned tensions detrimental for hard cemented carbide. In addition, the spherical basic shape holds a relatively small volume of cemented carbide.
Cemented carbide for rock drilling purposes generally contain WC, often referred to as alfa phase, and binder phase, which consists of cobalt with small amounts of W and C in solid solution, referred to as beta-phase. Free carbon or eta-phases, low carbon phases with the general formulas M.sub.6 C (CO.sub.3 W.sub.3 C), M.sub.12 C (Co.sub.6 W.sub.6 C) or kappa-phase M.sub.4 C are generally not present. However in EP-B2-0 182 759 cemented carbide bodies are disclosed with a core of fine and evenly distributed eta-phase embedded in the normal alpha+beta-phase structure, and a surrounding surface zone with only alpha+beta-phase. An additional condition is that in the inner part of the surface zone situated close to the core the binder phase content is higher than the nominal content of binder phase. In addition the binder phase content of the outermost part of the surface zone is lower than the nominal and increases in the direction towards the core up to a maximum situated in the zone free of eta-phase. With nominal binder phase content is meant here and henceforth weighed-in amount of binder phase.
In U.S. Pat. No. 5,286,549 cemented carbide bodies are disclosed, comprising WC(alpha-phase) and a binder phase based on at least one of Co, Fe and Ni and comprising a core of eta-phase-containing cemented carbide surrounded by a surface zone with an outer part of the surface zone having a lower binder phase content than the nominal, the binder phase content in the outer part of the surface zone being substantially constant. Cemented carbide bodies produced according to this invention have a high wear resistance because of a higher average hardness in the outer zone. Other related documents are U.S. Pat. No. 5,279,901 and EP-A-92850260.8. Cemented carbide bodies with a structure similar to EP-B2-0 182 759 are useful also as a punching or nibbling tool material as disclosed in U.S. Pat. No. 5,235,879 or as a roll material as in EP-A-93850023.8. Furthermore the material disclosed in U.S. Pat. No. 5,074,623 could also be used.
The object of the latter seven inventions (which are incorporated with the description by reference) is to achieve high wear resistance at the outer zone caused by the high hardness in combination with compressive pre-stresses caused by the different binder contents in the different zones. If the wear flat which develops during wear reaches the zone having a binder content higher than the nominal, the wear resistance is decreasing rapidly because of the lower hardness. This has been an disadvantage, in particular in rock drilling with insert-equipped bits.